Many telephones can be directly called because they have a unique telephone number associated with them. This telephone number is an address in the address space of the Public Switched Telephone Network. Many telephones cannot be directly addressed because they are accessible only via a private branch exchange. To call these telephones, you must first call the number for the private branch exchange and then a human operator or interactive voice response system asks for an extension.
The need exists therefore for a technique that enables the direct calling of telephones behind a private branch exchange.